Conventionally, as a method for growing seedlings of various plants, there is a transplant production method represented by a plant factory. This transplant production method is a method for stably growing high-quality uniform seedlings through labor-savings at a low cost by using a closed-type transplant production system including an artificial light source, air conditioner and an automatic irrigation unit to thereby artificially control light quantity, temperature, humidity, wind speed and irrigation quantity in a transplant production space to optimum states.
As this type of closed-type transplant production system, an artificial environmental system is disclosed in Japanese Patent No. 3026253. In this system, an air conditioning chamber is formed inside of a ceiling wall of a box-shaped outer chamber constituted by a thermal insulating material, a blowing chamber and a suction chamber are formed inside of opposed side walls of the outer chamber, respectively, and transplant production boxes are removably disposed in a multi-layer manner between the blowing chamber and the suction chamber. Air in the system is blown into a transplant production space from a honeycomb-structural wall of the blowing chamber and sucked by passing through a porous-plate wall of the suction chamber, and sent to the blowing chamber again by passing through a ventilation flue in the air conditioning chamber to thereby circulate the air. This circulation air is adjusted in terms of temperature and humidity by an air conditioner and blower positioned in the air conditioning chamber and circulated. However, in such a system described above, since the air conditioning chamber, the blowing chamber and the suction chamber are formed inside of the outer chamber, there is a problem in that utilization efficiency of transplant production space in the outer chamber is deteriorated. Also, since special rectifying structure for uniformly blowing air from the blowing chamber is used, structure of the system becomes complex.
Moreover, as an automatic irrigation unit used for this type of transplant production system, there is a unit disclosed by a report entitled, “Development of an injection type sub-irrigation unit for plug tray”, presented in a joint meeting of three scientific societies, namely, The Society of Agricultural Meteorology of Japan, Japanese Society of Environment Control in Biology and Japanese Society of High Technology in Agriculture, in 1999. The automatic irrigation unit reported here injects proper amounts of water and a culture solution to a culture medium for a short time by inserting a plurality of nozzles into a plug tray from bottom holes thereof. This irrigation unit has a feature that excess water or excess culture solution is not discharged because injected water does not leak from the bottom holes of the plug tray. It is necessary in such an irrigation unit, however, to prepare a large number of nozzles to be inserted into all of the bottom holes formed on bottom walls of tens to hundreds of plugs for a single plug tray, mechanically insert these nozzles into all of the bottom holes, and then inject an equal amount of water from each of these nozzles. Thus, in order to realize these requirements, there is a defect in that a complex and expensive mechanism is required.
Further, as another automatic irrigation unit, there is a unit disclosed by a report entitled, “Simplification of an automatic irrigation unit on the basis of evapotranspiration measurement of plug seedlings population”, presented in a joint meeting of The Society of Agricultural Meteorology of Japan and Japanese Society of Environment Control in Biology in 2000. In this automatic irrigation unit, an amount of evapotranspiration of a plant body and a culture medium is measured as a change in seedling population weight for each plug tray by placing the plug tray on a pan balance, a switch contact point is set to a pointer of the balance, and the switch contact point directly detects movement of the pointer to designate start of irrigation to the seedling population. This unit has a feature in that irrigation using a proper amount of water can be conducted without discharging excess water, since irrigation is started on the basis of the amount of evapotranspiration and irrigation using a minimum necessary amount of water being performed by using a subtimer. However, this report reveals that, since operation of the pointer has a mechanical resistance and movement of the pointer is directly influenced by gravity, operation of the pointer is incomplete or operational accuracy thereof has a problem.
Furthermore, Japanese Patent Laid-Open Specification No. 2001-346450 discloses a sub-irrigation unit, i.e. a watering unit capable of watering from a bottom of a plug tray, for use in a transplant production system with multi-layer shelving in a closed space. This sub-irrigation unit is provided with a shallow quadrangular box having three sides surrounded by side walls and having a bottom wall face. A drainage groove is formed at a side of the box having no side wall. A water supply pipe is disposed on a side wall face of the side of the box opposed to the drainage groove. A porous sheet of a synthetic resin is put on a bottom wall face of the box and plug trays are mounted on the porous sheet. According to the sub-irrigation unit having the above-described structure, water supplied from the water supply pipe is absorbed by the porous sheet due to its capillary action and spreads entirely to the bottom wall face of the box in a short time to thereby attain a water pool state at a predetermined water level and uniformly supply water to culture media contained in respective plugs from plug holes formed at a bottom of the respective plugs arrayed in the plug tray due to a capillary phenomenon. Since the culture medium in each plug comes into a water saturated state in a short time due to the capillary phenomenon, it is not necessary to maintain the pool state for a long time. However, unless a pump having a large discharge quantity is used, water does not spread to the bottom wall face of the box in its entirety and therefore a pool state is not realized. After irrigation is stopped, water remaining in the porous sheet is discharged to the drainage groove from an end of the porous sheet hanging down into the drainage groove. However, since the bottom of each plug contacts the porous sheet even after irrigation is stopped, the vicinity of the plug hole is easy to maintain in a wet state. As a result, roots of seedlings extend to outside from the plug hole, and therefore a problem occurs in a removal operation of seedlings from the plugs, and there is a danger of damaging the roots. To prevent the roots of seedlings from extending up to the vicinity of the plug hole by drying the vicinity of the plug hole after irrigation is stopped, it is proposed to form a plurality of small protrusions on the plug bottom so that the plug bottom does not directly contact with the porous sheet. However, a satisfied dry state is not always obtained.